A well blow out occurred in an underground storage in salt cavities holding US strategic reserves of crude oil, during special operation on one of the wells in the no. 6 cavity. The total capacity of the site is 8 Mm3 and cavity n°6, which is large, has several wells.

The well Completion comprises a 32.4 cm casing cemented to a depth of 816 m. A 24.4 cm pipe, 807 m long, is cemented inside. This pipe was probably added after the “initial” completion to improve oil tightness when the brine production cavity was being converted for storage. A 14 cm pipe had been used to withdraw the brine when oil was pumped in. The work on the well consisted of withdrawing the 14 cm pipe, repairing a leak on the casing, and reinforcing the wellhead equipment.

In order to withdraw the 14 cm pipe, the annular space between it and the 24.4 cm pipe had been filled with high viscosity mud to bring the pressure at the wellhead to zero. Then, a packer was set at the bottom of the 14 cm pipe to seal it off from the cavity. Work commenced on pulling the 14 cm pipe; however, after 14 lengths had been removed, the packer slipped, and the oil pushed it up to the surface. As the packer moved upward, the pressure differential on it increased. The packer then shot up to the surface, and the oil geyser continued until all the pressure was dissipated. An estimated volume of 10 000 m3 shot up into the air and caught fire, killing one man in the drilling crew. The total cost of the accident were estimated around $16 M.

The basic cause was the delicate operation that was undertaken while the oil was under high pressure and liable to expand violently if any mishap occurred. Although injecting the viscous mud in the annular space was a good precaution against any failure of the topside valve on the oil-filled annular space, it had no effect on the dangerous situation at the bottom of the cavity.

A more comprehensive precaution could be taken by releasing the pressure on the oil so that the pressure at the top of the annular space is removed. This would cause the air/brine interface in the central tubing to drop by about one quarter of the total height. The volume removed would, of course, be exactly equal to the volume that would be expelled in an accident. As well head oil pressure is zero, no blow out can take place.